In a turbocompressor, one or more impellers are directly connected to a shaft. For their operation, the impellers must be driven at very high rotational speeds, e.g. 20000 rpm up to 100000 rpm and even more. Traditionally, these elevated speeds were attained by combining a standard induction motor and a gearbox, the latter consisting of a large bull gear and at least one small pinion gear. The losses in this gearbox can be considerable, negatively influencing the system efficiency. Moreover, such a gearbox is heavy and it constitutes a major part of the footprint of the entire system.
Meanwhile, advances in high speed motor technology have enabled the development of direct driven turbocompressors. By increasing the motor speed, less torque is required for the same output power. However, since the motor volume is known to vary approximately proportional to the torque, this also implies a higher power and loss density. Consequently, the operational limits of high speed motors and direct driven turbocompressors are strongly determined by the performance of the cooling system and by the extent to which losses, are kept under control.
Many techniques for cooling electrodynamic machines have been published before. Obviously, these ideas have been adopted in the field of direct driven turbomachines. Below, a summary of these methods is given, for radial flux machines with an inner rotor.
Providing cooling fins at the exterior of a machine is standard. Their operation can be enhanced by some way of forced convection using an independent fan or even, in case of a compressor, by ducting the process gas partially or entirely over these fins. In the scope of direct driven turbocompressors, this is found e.g. in U.S. Pat. No. 6,675,594 B2, KR 10/0572849 B1 and KR 10/0661702 B1.
The use of cooling channels or shells in a housing surrounding the stator, through which a fluid—most frequently a liquid—is flowing, is common. If properly designed, copper and iron losses generated in the stator can be efficiently evacuated in this way. Though apparently simple, there are some constructional issues that require particular attention. For example, the cooling channels are often to be sealed properly from the rest of the system. In systems where the cooling fluid is directly in contact with the stator outer circumference, leakage towards the inside of the stator is undesired. If the latter is prevented by a thin supplementary shell in between the stator lams and the cooling channels, an additional thermal contact resistance is introduced. This type of cooling is encountered in many patents dealing with direct driven turbomachines, as e.g. in U.S. Pat. No. 5,605,045 A, U.S. Pat. No. 5,857,348 A, U.S. Pat. No. 6,296,441 B1, U.S. Pat. No. 6,579,078 B2, U.S. Pat. No. 6,675,594 B2, U.S. Pat. No. 6,685,447 B2, U.S. Pat. No. 7,160,086 B2, U.S. Pat. No. 7,240,515 B2, US 2007/269323 A1, U.S. Pat. No. 7,338,262 B2, U.S. Pat. No. 7,367,190 B2, KR 10/0572849 B1, WO 00/17524 A1, WO 00/49296 A1 and WO 2008/138379 A1. However, plenty of prior art on this cooling technique is found in patents only dealing with motor or generator cooling as such: e.g. in U.S. Pat. No. 3,184,624 A, U.S. Pat. No. 3,480,810 A, U.S. Pat. No. 3,567,975 A, U.S. Pat. No. 4,516,044 A, U.S. Pat. No. 4,700,092 A. In US 2003/038555 A1 and U.S. Pat. No. 6,507,991 B1, this concept is applied to a slotless motor design, where the cooling channels are formed by radial outward fins integrated in the core.
Exterior cooling channels or shells are not always sufficient to get a system thermally under control. Sometimes, the stator and/or the coils are therefore equipped with internal axially oriented cooling channels. These may be completely sealed from their surrounding, allowing e.g. cooling with special agents. They may as well be open, thereby enabling a fluid to cool other structures such as the endturns as well. In the field of turbomachines, this is seen e.g. in U.S. Pat. No. 6,471,493 B2, US 2008/253907 A1, WO 00/49296 A1, WO 2007/110281 A1 and EP 1680855 B1.
The previous methods mainly focus on evacuating heat generated inside the stator core and the coils. Heat produced in more deeply lying system elements, such as the rotor and the gap (windage loss), is hardly evacuated. Therefore, one frequently encounters methods in which a gas—often air—is forced to flow in some way through the gap between the stator and the rotor. Several gas flow configurations can be distinguished: gas entering the gap at one axial end and leaving the gap at the opposite axial end, gas entering/leaving the gap at both axial ends and leaving/entering the gap radially through the coils and/or the stator thereby cooling these structures as well. In the field of direct driven turbomachines, this cooling method is encountered in e.g. U.S. Pat. No. 6,579,078 B2, U.S. Pat. No. 6,994,602 B2, U.S. Pat. No. 7,160,086 B2, WO 95/08861 A1, WO 2007/110281 A1 and WO 2008/138379 A1. Pal et al., cfr. US 2007/018516 A1, additionally apply a sort of labyrinth structure in between the rotor and the stator to enhance cooling. The idea of cooling a motor by a forced gas flow through the gap between the stator and the rotor is found as prior art in earlier patents dealing with motor cooling alone, as e.g. in U.S. Pat. No. 3,110,827 A, U.S. Pat. No. 4,544,855 A or GB 772973 A.
The required pressure for driving gas through the gap and/or its surrounding structure, may originate from a separate fan, from a small blower directly attached to or integrated in the shaft, or even from a tap after the first compression stage, in case of a compressor. Kim et al., cfr. KR 2001/0064011 A, have integrated a sort of blower inside the active part of the motor, using a thin helical groove at the stator inner surface or the rotor outer surface. The axial pressure drop is influenced by tangential acceleration of the gas while entering the gap. It is of particular concern in high speed motors, where the tip speed of the rotor may be extremely high. In such cases, relatively large powers may be required to achieve this type of forced convection, thereby reducing the efficiency of the whole system. This negative impact can be reduced by increasing the gap or by leaving some free space on top of the slots. Obviously, both suggestions affect the electro-magnetic design as well.
If necessary for the application, the inside of the rotor can also be cooled by a forced flow of some fluid or liquid through a particular configuration of axial and/or radial holes, as seen e.g. in U.S. Pat. No. 5,605,045 A, U.S. Pat. No. 6,296,441 B1, U.S. Pat. No. 6,685,447 B2 and GB 2454188 A.
In addition to the iron, copper and windage losses inside the machine, a large part of the copper loss is generated in the endturns. These can be cooled by forced convection as well. This may be done independently or in combination with one of the previous methods. Explicit examples of this method related to turbomachines are found in e.g. U.S. Pat. No. 6,009,722 A, U.S. Pat. No. 6,471,493 B2, U.S. Pat. No. 6,675,594 B2, U.S. Pat. No. 7,160,086 B2, US 2008/253907 A1, WO 00/49296 A1, KR 2001/0064011 A, KR 10/0661702 B1 and WO 2008/138379 A1. Prior art is e.g. found in U.S. Pat. No. 3,932,778 A, U.S. Pat. No. 4,246,503 A, U.S. Pat. No. 4,306,165 A and CH 397844 A.
Another method for cooling the endturns is pouring them into an electrically insulating yet thermally conductive material, in order to realize a thermal bridge towards another thermally conductive material, often the machine's housing. Prior art is e.g. found in U.S. Pat. No. 4,128,527 A, U.S. Pat. No. 4,492,884 A, U.S. Pat. No. 6,201,321 B1 and U.S. Pat. No. 6,445,095 B1.
Thus far, only techniques for evacuating losses have been discussed. However, designers should first try to keep the losses of the entire system as small as possible. This is especially true for high speed motors, for their high power and loss density. Below, some alternative choices are discussed.
One may e.g. choose between different motor types. Electronically commutated (EC) machines excited with permanent magnets, such as permanent magnet synchronous machines (PMSM) and brushless DC machines (BLDC), rotate at the same speed as that of the applied magnetic field. The generated rotor losses are basically due to stator slotting and/or current harmonics. Their value is relatively small and their presence is not fundamental for correct operation of the machine. In an induction machine, additional losses are generated by the currents induced in the rotor due to the slip. Also, the efficiency of induction motors is more sensitive to speed variations, making them less attractive in applications where the speed is to be controlled over a relatively large range.
In a high-speed context, characterized by relatively low torques, the slip of an induction machine can be very small, and other properties may determine the choice of motor type. E.g. Induction machines are known for their relatively low cost and ease of operation, whereas PMSM machines with surface mounted magnets contained within a sleeve are fairly complicated and thus more costly. On the other hand, when aiming high speed operation, rotordynamic considerations generally urge the use of solid rotors even in case of induction machines, requiring other design approaches for these machines.
Other motor types such as switched reluctance motors and traditional DC motors are less suitable for high-power high-speed applications and are therefore not considered in the discussion.
Complementary to distinguishing between motor types based on their operation principles, one can choose between motors based on their stator construction. The majority of radial flux motors with an inner rotor have a slotted stator. Coils can thereby be concentrated around a single tooth or distributed over the stator. Concentrated windings are much easier to insert than distributed windings, but their resulting spatial distribution of the magneto-motive force causes more harmonic losses and cogging in the machine. This makes concentrated windings less suitable for high-speed applications.
However, even when equipped with a distributed winding, a slotted design induces more losses in the rotor than a slotless design does, for the latter has a much larger magnetic gap between stator and rotor. Slotless machines also exhibit less cogging for the same reasons. This makes slotless machines attractive for high-speed applications, particularly when combined with a permanent magnet rotor. A thorough investigation of slotless permanent magnet high speed motors is found in the PhD thesis of Jörgen Engström, “Analysis and Verification of a Slotless Permanent Magnet Motor for High Speed Applications”.
Another way for controlling the loss in a motor is choosing between different core materials. If laminated steels are used, one can minimize the eddy current loss by maximally reducing the thickness of the lams, even though this may significantly increase the material cost. Next to thickness, the grade selection plays a crucial role in loss minimization. Both, non-oriented and oriented low-loss grades are found in motors. The non-oriented grades are most common for their isotropic properties. However, the anisotropic magnetic nature of oriented steels should be exploited whenever possible. First, standard available grain-oriented steels have significantly lower specific energy losses than standard available non-oriented steels (e.g. 0.73→1.11 W/kg compared to 2.1→>8 W/kg, all values at 1.5 T peak and 50 Hz). Second, standard available grain-oriented steels are thinner than standard available non-oriented steels (e.g. 0.23→0.35 mm compared to 0.35→0.65 mm). Consequently, given a cost, frequency and flux density level, the use of grain-oriented steel favorably influences the machine's efficiency. A thorough discussion of electrical steel properties is given in the book “Electrical Steels for Rotating Machines”, by Philip Beckley.
One could use soft magnetic composite (SMC) powders as well. These are attractive for high speed applications, for their comparatively low eddy current loss at elevated frequencies. They can also be advantageously used in unconventional motor configurations, for their 3D isotropic magnetic and thermal properties. On the other hand, their permeability and saturation flux density is smaller than that of traditional motor steels, and small series production with SMC is not likely to be cost-effective. Thorough descriptions of the applicability of SMCs in electrical motors is e.g. found in the publications “Soft magnetic composites offer new PM opportunities” by Persson et al, “Comparative Study of. High-Speed PM Motors with Laminated Steel and Soft Magnetic Composite Cores” by Yunkai Huang et al. and “Experience with ATOMET Soft Magnetic Composites Properties, Pressing Conditions and Applications” by Viarouge et al., amongst many others.
One could also consider using amorphous or even nanocrystalline ribbons. However, since these are very thin and hard, fabrication tools and dies wear more rapidly, increasing the cost of such stators. Moreover, these materials are brittle and feature significant magnetostriction. Hence, magnetic stators built with this type of material, are subject to large stresses varying at multiples of the rotational frequency, most likely limiting their lifetime. Particular measures should therefore be taken when used in motors. Nevertheless, some applications have e.g. been found in U.S. Pat. No. 4,255,684 A, U.S. Pat. No. 6,737,784 B2 and U.S. Pat. No. 6,960,860 B1.
In order to further reduce the motor loss, one may take some measures outside the motor as well. One common example is a sine filter. This device filters the higher harmonics in the motor current, which would otherwise cause non-synchronous rotating fields and hence extra losses in the machine.
Another exterior example is the power electronic drive, basically consisting of a set of semiconductors that are continuously switching between on and off, according to some particular control scheme. The smaller the switching frequency is, the larger the harmonic content of the output current is, and the higher the motor losses are. Obviously, the particular switching pattern itself (e.g. sinusoidal PWM, space vector modulation, etc. . . . ) affects the losses as well, in addition to the number of phases and/or poles of the motor.
From the previous non-exhaustive description, it follows that a sound thermal management of a high speed motor can only be the result of a well-considered combination of several of the mentioned measures. Actually, the designer of a high speed motor should make his decisions on a higher level than that of the motor alone. In case of a compressor unit, one of the major points of interests is the total system efficiency. In that respect, it is not sufficient to get the losses in some parts below a certain value. It should also be done efficiently, at an acceptable cost and in view of the entire system.
For example, large switching frequencies in the semiconductors might be attractive for the motor, but they reduce the efficiency of the drive. Hence, some trade-off must be found. In the particular case of high speed motors, this may become a difficult exercise, since the base frequency of the machine may be so high as to require elevated switching frequencies anyhow for still getting some acceptable output current harmonics level.
In another example, cooling gas might be conducted through the gap and/or some ducts in the machine. Whether the required pressure is obtained via a separate fan, via a small blower directly attached to or integrated in the shaft or from a tap after the first compression stage, this involves some power consumption and hence influences the overall system efficiency.
In some applications, the process gas is entirely guided over a series of fins or through a sort of cooling shell around the stator before being compressed. Doing so avoids additional equipment but also entails some efficiency loss on the system level, because the entailed pressure loss must be compensated by a higher pressure ratio in the stage(s) and because the gas is already heated prior to compression. Nevertheless, it is found e.g. in U.S. Pat. No. 6,009,722 A, U.S. Pat. No. 6,675,594 B2, U.S. Pat. No. 6,774,519 B2, WO 00/49296 A1, WO 02/50481 A1, KR 10/0572849 B1, KR 10/0661702 B1.
This invention relates to radial flux slotless motors for turbocompressors. Therefore, an overview of state-of-the-art radial flux slotless motor technology is given.
Radial slotless motors can be distinguished by their particular coil and core construction. Often, the coil concept is rather traditional, in the sense that the iron core entirely surrounds the copper conductors. Among this class, one can make a further distinction between coils approximating a homogeneous fill of the space between the rotor and the stator iron, and coils that do not have this property. However, many slotless designs are encountered in which a coil is toroidally wound around the core. In such cases, copper is being found at both radial sides of the core. In this class, one could distinguish between designs having a stator core constructed as a single piece or designs have a stator core consisting of a series of segments.
Examples of radial slotless motors having an iron core entirely surrounding a homogeneously distributed set of conductors are found e.g. in patents U.S. Pat. No. 4,211,944 A, U.S. Pat. No. 5,197,180 A, U.S. Pat. No. 5,313,131 A, U.S. Pat. No. 5,998,905 A, U.S. Pat. No. 6,072,262 A, U.S. Pat. No. 6,507,991 B1, US 2003/038555 A1, U.S. Pat. No. 6,806,612 B2, U.S. Pat. No. 7,269,890 B2, US 2007/269323 A1, WO 02/15229 A1, WO 2004/098025 A1, WO 2008/085466 A1, EP 0653112 B1, CA 1136200 A1, JP 8154350 A, JP 2002/325404 A, JP 2002/345217 A, JP 2005/110454 A and JP 2006/288187 A. For further reference, it is indicated that the slotless motor concept shown in US 2003/038555 A1 and U.S. Pat. No. 6,507,991 B1 also belongs to this class, but has a segmented core consisting of two arc-shaped parts as well.
Examples of radial slotless motors having an iron core entirely surrounding a non-homogenously distributed set of conductors are found e.g. in patent documents U.S. Pat. No. 4,563,808 A, U.S. Pat. No. 4,818,905 A, U.S. Pat. No. 6,894,418 B2, U.S. Pat. No. 7,084,544 B2, WO 91/01585 A1, WO 00/07286 A1, EP 1680855 B1, GB 2429849 B, JP 2001/333555 A, JP 2002/272049 A, JP 2003/102135 A, JP 2005/110456 A, JP 2007/014140 A, JP 2007/135392 A, JP 2007/336751 A, RU 2120172 C1 and RU 2206168 C2. The differences between all these concepts are generally related to the construction of both the coils and the core, and the materials that are used for the core.
Examples of radial slotless motors having a toroidal coil wound around a stator core constructed as a single piece are found e.g. in U.S. Pat. No. 4,103,197 A, U.S. Pat. No. 4,547,713 A, U.S. Pat. No. 4,563,606 A, U.S. Pat. No. 5,304,883 A, U.S. Pat. No. 5,455,470 A, U.S. Pat. No. 6,242,840 B1, U.S. Pat. No. 6,344,703 B1, U.S. Pat. No. 6,989,620 B2, U.S. Pat. No. 7,145,280 B2, US 2008/018192 A1, U.S. Pat. No. 7,391,294 B2, WO 00/49296 A1, WO 2004/057628 A2, EP 1017151 A2, EP 0964498 B1, EP 1017153 B1, EP 1100177 A1, DE 3629423 A1, GB 2436268 B, JP 2008/048490 A, KR 2004/0065520 A and KR 10/0804810 B1. They are all characterized by an annular shape of the final core. They basically differ by the way the coils and the core are constructed.
Examples of radial slotless motors having a toroidal coil wound around a stator core constructed as a series of segments have only been found in a few patents. Zhang, EP 1324472 A2, proposes an annular shaped core consisting of three arc-shaped segments. Choi, KR 2004/0065521 A, KR 2004/0065529 A and KR 2004/0065531 A, proposes a hexagonally shaped core of six linear segments. The latter is particularly attractive for constructional purposes, but it introduces a non-uniform gap and thus causes some cogging.
The idea of constructing a stator core out of segments is not new, particularly not in the field of very large motors or generators. Though, it can have some attractive advantages for smaller machines as well. For example, the rotor of high speed machines is often made out of a solid steel base. In such cases, the inner part of a punched lamination is to be scrapped. Using a segmented core yields significant material savings. Sometimes, segmentation is done for explicitly allowing the use of grain-oriented steels for its attractive properties as described earlier (U.S. Pat. No. 4,672,252 A, US 2006/043820 A1). In addition to the latter, further material savings can even be obtained when separately punching teeth and/or core segments (GB 1395742 A, US 2001/030486 A1, WO 01/05015 A2, WO 99/01924 A1, WO 01/34850 A2, DE 102004029442 A1). Segmentation may also be used to ease the insertion of coils and/or the entire manufacturing process (U.S. Pat. No. 5,986,377 A, U.S. Pat. No. 6,507,991 B1, U.S. Pat. No. 6,583,530 B2, U.S. Pat. No. 6,781,278 B2, U.S. Pat. No. 7,084,545 B2, U.S. Pat. No. 7,122,933 B2, US 2003/038555 A1, US 2005/269891 A1, EP 1322022 B1, EP 1901415 A1, GB 2394123 B, DE 102005055641 A1, JP 54041401 A, KR 2004/0065521 A).